drivers/xen: make xenbus_dev_[front/back]end explicitly non-modular

The Makefile / Kconfig currently controlling compilation here is:

obj-y   += xenbus_dev_frontend.o
[...]
obj-$(CONFIG_XEN_BACKEND) += xenbus_dev_backend.o

...with:

drivers/xen/Kconfig:config XEN_BACKEND
drivers/xen/Kconfig:    bool "Backend driver support"

...meaning that they currently are not being built as modules by anyone.

Lets remove the modular code that is essentially orphaned, so that
when reading the driver there is no doubt it is builtin-only.

Since module_init translates to device_initcall in the non-modular
case, the init ordering remains unchanged with this commit.

We also delete the MODULE_LICENSE tag since all that information
is already contained at the top of the file in the comments.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>